class Vector2D {
private:
    vector<vector<int>>::iterator row, iBegin, iEnd;
    vector<int>::iterator col;
public:
    Vector2D(vector<vector<int>>& vec2d) {
        iBegin = row = vec2d.begin();
        iEnd = vec2d.end();
        if(vec2d.size())
            col = row->begin();
        //remove();
        //col++;
        //col++;
        /*
        if(hasNext())
        cout<<next()<<endl;
        if(hasNext())
        cout<<next()<<endl;

        if(hasNext())
        remove();
        row = iBegin;
        col = row->begin();
        */
    }

    int next() {
        if(hasNext()){
            int val = *col;
            col++;
            return val;
        }
    }

    bool hasNext() {
        while(row != iEnd && col == row->end()){
            row++;
            col = row->begin();
        }
        return row != iEnd;
    }

    bool remove(){
        if(col == row->begin()){
            auto pre = prev(row);
            while(pre != iBegin && (*pre).empty()) pre = prev(pre);
            //cout<<"remove sz"<<(*pre).size()<<endl;
            if(!(*pre).empty()) {
                cout<<"remove "<<*prev((*pre).end())<<endl;
                (*pre).erase(prev((*pre).end()));
            }
        }
        else{
            (*row).erase(prev(col));
            col--;
        }
        return true;
    }
};

/**
 * Your Vector2D object will be instantiated and called as such:
 * Vector2D i(vec2d);
 * while (i.hasNext()) cout << i.next();
 */